Creating model of disc with offset hole for 3D printing
I just had a look at the file and I can see disconnected geometry around the top and bottom rings:



Trying creating it in a similar fashion to this:

Add a circle mesh and select Nothing on fill type.



Tab to go into edit mode.  Select all by pressing A.  Press E to extrude and press Enter without moving the mouse.  Press S to scale in toward the center.  Then select the new hole and offset it by the proper margin.



Select all (A) and extrude again in the Z direction this time.  You now have a hole with faces:



Another way to do this with a little more precision is with the knife tool (you can find tutorials on this forum).  But after you make your cut, and delete the faces, you must Bridge Edge Loops by pressing W: